Design of a spent-fuel assay device using a lead spectrometer

The neutron slowing-down-time method for nondestructive assay of light wate r reactor spent fuel has been under development for many years. Results for a newly optimized design of a lead slowing-down-time spectrometer for spen t-nuclear-fuel assay are presented. Monte Carlo analyses were performed to optimize the design of the assay device, determine its main parameters, inv estigate the effects of the spent-fuel assembly and the detector impurities on its performance, determine the fission signatures of the fissile isotop es in spent-fuel elements, and simulate the assay signal as a function of t he slowing-down time, assuming threshold fission chambers for the assay det ectors. The assay signals from the threshold detectors were analyzed to pre dict the unknown masses of the fissile isotopes in a typical spent commerci al light water reactor fuel element. The broadened resolution of the system caused by the presence of the spent fuel inside the spectrometer pile was found sufficient to separate the signatures of the U and Pu fissiles in spe nt fuel.